The generation of immunity requires activation of T lymphocytes. Ly-6A.2 is involved in T cell responses and abnormalities in its expression have been noted in several autoimmune disease models. This proposal investigates the role of Ly-6A.2 in vivo and its mechanism of action. Understanding the function of Ly-6A.2 may lead to approaches to manipulate immune responses and may provide insight into autoimmune diseases. This proposal has 4 Aims: 1. Define the structure and expression of the ligand for Ly-6A.2. One central hypothesis of this project is that Ly-6A.2 is a lymphocyte receptor that binds a ligand expressed on the surface of accessory cells. The goal of this Aim is to define this ligand biochemically and to determine its pattern of expression. The primary approach to identify the ligand uses a soluble recombinant Ly-6A.2-IgG1 fusion protein. These experiments will elucidate the basic molecular features of the ligand. In addition, these studies will define where and under what conditions Ly- 6A.2 is potentially engaged by its ligand, which should give insight into the function of Ly-6A.2. 2. Molecularly clone the ligand for Ly-6A.2. The goal of this Aim is to isolate and characterize a cDNA clone that encodes the ligand for Ly-6A.2. The experimental approach will be to probe cDNA expression libraries with anti-ligand reagents or to probe libraries with oligonucleotides based on the amino acid sequence. These experiments will allow us to definitively establish the molecular identity of the Ly-6A.2-ligand and to determine relationships with known genes. Of equal importance, the cDNA will provide a key reagent for transfection experiments to further probe the structure and function of the receptor-ligand interaction. 3. Define the function of the Ly-6A.2-ligand. The goal of this set of experiments is to define the functional consequence of Ly-6A.2<->ligand interactions. The experimental approach will be to test the effects of stimulating or costimulating T cells with recombinant Ly-6A.2-ligand and also to test the effect on T cell responses of blocking this molecule with anti-ligand reagents. Another approach will examine whether Ly-6A.2 interaction with its ligand contributes to T-APC adhesion. These results will give further insight into the role of this receptor-ligand system in immune responses. 4. Study the role of Ly-6A.2 using animal models with altered gene expression. The goal of this Aim is to address: (i) The role of Ly-6A.2 in T cell development. The underlying hypothesis is that developmental regulation of Ly-6A.2 is important for the normal maturation of thymocytes; (ii) The role of Ly-6A.2 in primary T lymphocytes and in immune responses in vivo. The experimental approach to both questions will be to study the effects of altering the expression of Ly-6A.2 in development and/or in peripheral T cells using transgenic mice.